This invention relates to an exposure control system for use in such a copying machine as an electric copying machine, a graphic arts copying machine or the like that can automatically control the exposure time.
Usually, in a copying machine in which a manuscript and an image focusing plane are held stationary, light is projected upon the surface of the manuscript from an exposure light source and the image of the manuscript is focused on the image focusing plane through a projection lens to form a copy. Where it is desired to vary the magnifying power, the projecting lens is moved along the optical axis thereof and at the same time, for the purpose of varying the total optical path length, a reflection mirror interposed between the manuscript and the projecting lens is also moved to obtain correct focusing.
For this reason, as the magnifying power is increased, the distance between the projecting lens and the focusing plane is also increased, thus decreasing the quantity of light incident upon the focusing surface. On the other hand, decrease in the magnifying power decreases the distance between the projecting lens and the focusing surface, thus increasing the quantity of light incident upon the focussing surface.
The projecting lens and the reflection mirror are moved in parallel with the optical axis inside the manuscript support, but as the optical axis between the manuscript and the reflection mirror is moved across the center of the manuscript surface the image focusing position is displaced from the center of the focusing plane with the result that the distribution of the quantity of the incident light upon the focusing plane becomes nonuniform due to the cos.sup.4 law of lens, thereby making nonuniform the light distribution of the focused image.
To obviate this difficulty, a voltage phase control system has been used in which after adjusting the light quantity of the light source, the light source is lighted for a definite interval. However, as pointed out above, since copying is made with an incident light distribution which becomes nonuniform due to illumination based on the cos.sup.4 law of lens, it has been impossible to eliminate nonuniformity of the exposure light quantity due to this maldistribution. Moreover, with this voltage phase control system, it has been impossible to obviate electrical noise caused by the variation in the source voltage waveform.